The present invention relates to a process for the continuous preparation of polybutylene terephthalate (PBT) from terephthalic acid (TPA) and 1,4-butanediol (BDO).
The preparation of polybutylene terephthalate from dimethyl terephthalate (DMT) and 1,4-butanediol is known from the prior art. A disadvantage of this process is that tetrahydroftiran (THF) formed in small amounts as by-product forms an azeotrope with the methanol liberated during the reaction and therefore can be recovered as a material of value only with great difficulty.
The direct preparation of polybutylene terephthalate from terephthalic acid and 1,4-butanediol is made difficult by the formation of relatively large amounts of THF, resulting in loss of 1,4-butanediol required for the reaction. Furthermore, not only THF but also 2,5-dihydrofuran (2,5-DHF) is formed from 1,4-butanediol. The 2,5-dihydrofuran is difficult to separate from THF and therefore contaminates and reduces the quality of the valuable product THF. A further problem in the direct preparation of polybutylene terephthalate from terephthalic acid and 1,4-butanediol is that terephthalic acid is not soluble in 1,4-butanediol and goes into solution only during the esterification with 1,4-butanediol. However, to produce high quality polybutylene terephthalate, it is extremely important for this to be free of contaminants such as free acid groups from terephthalic acid. For this reason, the terephthalic acid should be completely esterified and dissolved before the actual polycondensation commences.
Processes concerned with the direct preparation of polybutylene terephthalate from terephthalic acid and 1,4-butanediol are already known from the prior art. DD-A 269 296 relates to a continuous process for preparing polyalkylene terephthalates. Setting of appropriate reaction parameters in the esterification step of the dicarboxylic acid used with the glycol used is said to decisively favor removal of water from the esterification phase so that esterification products are obtained both with a high degree of conversion and a high mean degree of polymerization. The esterification step is carried out in a reactor cascade in which the temperature is increased and the pressure is reduced from reactor to reactor. The example described relates to the preparation of polyethylene terephthlate from terephthalic acid and ethylene glycol.
EP-A 0 431 977 describes a process for increasing the direct esterification rate of a diacid and 1,4-butanediol to esterification of  greater than 95% of the acid groups. The process can be carried out continuously in three reactors. The process described comprises:
a) mixing 1,4-butanediol and diacid in a ratio of at least 2:1,
b) heating the reaction mixture to 180xc2x0 C.,
c) adding a suitable catalyst and
d) reacting the mixture at atmospheric pressure and a mean temperature in the range from 180 to 245xc2x0 C. for a maximum of 60 minutes.
In this process, less than 5% of the 1,4-butanediol is said to be cyclized to THF. However, the content of free acid groups in the end product is high.
EP-A 0 046 670 relates to a process for preparing polybutylene terephthalate by direct esterification of terephthalic acid and 1,4-butanediol. The process comprises an esterification step at a temperature of up to 215xc2x0 C. and atmospheric pressure. After consumption of most of the terephthalic acid, at which stage, however, from 10 to 40% by weight of terephthalic acid is still present, i.e. before the clearing point, the polycondensation stage is carried out at a temperature higher than that in the esterification stage.
DE-A 27 11 331 describes the preparation of polyester oligomers by means of a two-stage esterification. Here, the first esterification stage is carried out at atmospheric pressure and the second esterification stage is carried out at atmospheric pressure or subatmospheric pressure, at a temperature of about 250xc2x0 C. in both stages. However, the examples describe only the reaction of terephthalic acid with ethylene glycol.
DE-A 35 44 551 relates to the continuous preparation of polybutylene terephthalate from terephthalic acid and 1,4-butanediol. The preparation is carried out in three stages. The first stage, namely the esterification, is carried out at from 225 to 260xc2x0 C. and a pressure of from 0.1 to 1 bar. The second stage, the precondensation, is carried out at from 230 to 260xc2x0 C. and a pressure of from 10 to 200 mbar, and the third stage, the polycondensation, is carried out at from 240 to 265xc2x0 C. and a pressure of from 0.25 to 25 mbar.
It is an object of the present invention to provide a process for preparing polybutylene terephthalate which is improved compared to the prior art. In particular, formation of THF from the 1,4-butanediol used should be as low as possible and the polybutylene terephthalate obtained should have a very low content of free acid groups.
The achievement of this object starts out from a process for the continuous preparation of polybutylene terephthalate from terephthalic acid and 1,4-butanediol, comprising:
a) direct esterification of terephthalic acid with 1,4-butanediol in a reactor cascade comprising at least two reactors,
b) precondensation of the esterification product obtained in stage a), and
c) polycondensation of the precondensate obtained in stage b).
In the process of the present invention, the temperature decreases along the reactor cascade in stage a).
The polybutylene terephthalate prepared according to the present invention is of excellent quality. It has a low content of acid and alcohol groups. The formation of THF and 2,5-DHF from 1,4-butanediol is low. As a result, only little 1,4-butanediol is lost, so that the yield of polybutylene terephthalate based on 1,4-butanediol is high.
a) Esterification Stage
The stage a) is carried out in a reactor cascade comprising at least two reactors, preferably from two to five reactors, particularly preferably three reactors. The reactors used are generally stirred vessels.
The temperature range for the overall esterification stage is generally from 170 to 250xc2x0 C., preferably from 180 to 240xc2x0 C., particularly preferably from 190 to 230xc2x0 C. According to the present invention, the temperature decreases along the reactor cascade, i.e. the esterification temperature drops from reactor to reactor. The temperature in an esterification reactor is generally from 2 to 30xc2x0 C. lower than that in the preceding reactor. The temperature preferably drops by from  greater than 5 to 30xc2x0 C. from reactor to reactor. In a preferred embodiment, the reactor cascade comprises three reactors, and the temperature in reactor 1 (T1) is generally from 200 to 250xc2x0 C., preferably from 210 to 240xc2x0 C., particularly preferably from 218 to 230xc2x0 C. The temperature in reactor 2 (T2) is generally from 190 to 230xc2x0 C., preferably from 200 to 225xc2x0 C., particularly preferably from 205 to 220xc2x0 C., and the temperature in reactor 3 (T3) is generally from 170 to 220xc2x0 C., preferably from 180 to 215xc2x0 C., particularly preferably from 190 to 210xc2x0 C., with the temperature dropping from reactor to reactor.
The residence times for the overall esterification stage are generally from 140 to 430 minutes, preferably from 160 to 420 minutes, particularly preferably from 170 to 390 minutes. In the case of a reactor cascade made up of three reactors, the residence time in the first reactor (V1) is generally from 100 to 250 minutes, preferably from 110 to 250 minutes, particularly preferably from 120 to 240 minutes, in the second reactor (V2) generally from 20 to 105 minutes, preferably from 30 to 100 minutes, particularly preferably from 30 to 90 minutes, and in the third reactor (V3) generally from 20 to 75 minutes, preferably from 20 to 70 minutes, particularly preferably from 20 to 60 minutes.
The esterification stage is generally carried out at a reaction pressure of not more than 1 bar. Preference is given to a pressure of  less than 1 bar. The experimental parameters pressure (p) and temperature (T) in the respective reactor particularly preferably lie within a plane defined by
p(lower limit)=0.348xc3x97(T/xc2x0 C.)2xe2x88x92124.12xc3x97(T/xc2x0 C.)+11121 and
p(upper limit)=0.0802xc3x97e(0.0405xc3x97(T/xc2x0 C.)),
where the pressure does not increase from reactor to reactor. Very particularly preferably, the pressure drops continuously from reactor to reactor. In the case of a reactor cascade made up of three reactors, the pressure in the first reactor (p1) is preferably from 650 to 900 mbar, in the second reactor (p2) preferably from 500 to 600 mbar and in the third reactor (p3) preferably from 350 to 500 mbar. The pressure difference between the individual reactors is generally at least 50 mbar, preferably from 50 to 400 mbar, particularly preferably from 100 to 300 mbar.
The preferred process conditions with a pressure of less than 1 bar suppress the formation of THF from 1,4-butanediol even more effectively. Furthermore, only very small amounts of 2,5-dihydrofuran (2,5-DHF), which arises as a further product from 1,4-butanediol, are formed. The 2,5-dihydrofuran is difficult to separate from THF by distillation and therefore contaminates the valuable product THF and reduces its quality.
The esterification is generally carried out using a molar excess of 1,4-butanediol in order to push the ester equilibrium in the desired direction. The molar ratios of 1,4-butanediol to terephthalic acid are generally from 1.1:1 to 3.5:1, preferably from 1.5:1 to 2.8:1, particularly preferably from 1.9:1 to 2.5:1.
In a preferred embodiment, a suspension comprising 1,4-butanediol and terephthalic acid in a molar ratio of generally  less than 2:1, preferably  less than 1.5:1, is placed in a reservoir and diluted with hot 1,4-butanediol so as to heat it to from 50 to 100xc2x0 C., preferably from 60 to 100xc2x0 C., particularly preferably from 70 to 90xc2x0 C., and give a ratio of 1,4-butanediol to terephthalic acid corresponding to the abovementioned final ratio.
An esterification catalyst, generally a Lewis acid metal compound, preferably of titanium or tin, is added to this BDO/TPA mixture. Particularly preferred esterification catalysts are tetrabutyl orthotitanate (TBOT), triisopropyl titanate and tin dioctoate, with very particular preference being given to tetrabutyl orthotitanate. The catalyst is generally used in the esterification stage in an amount of  less than 200 ppm, preferably from 65 to 150 ppm, particularly preferably from 75 to 100 ppm, calculated as the metal of the esterification catalyst used and based on polybutylene terephthalate. All of the catalyst can be added to the first reactor. However, in a preferred embodiment only part of the catalyst, preferably  less than 50 ppm, particularly preferably  less than 25 ppm, calculated as the metal and based on polybutylene terephthalate, is introduced into the first reactor and the remainder of the catalyst is introduced into the subsequent reactors, preferably into the second reactor. The esterification catalyst is preferably introduced into the reactor as a mixture with 1,4-butanediol.
The reaction mixture comprising terephthalic acid, 1,4-butanediol and an esterification catalyst is reacted in a reactor cascade to a conversion of generally  greater than 97%, preferably from 97 to 99%, based on terephthalic acid. If the esterification stage is carried out in a reactor cascade having three reactors, the esterification in the first reactor generally proceeds to a conversion (C1) of  greater than 89%. The THF/water mixture formed is separated off and the reaction mixture is transferred to the second reactor in which it is esterified to a conversion (C2) of generally  greater than 95%. At this point in time, all of the terephthalic acid has generally reacted or gone into solution, which can be seen from a clear reaction mixture (clearing point). The reaction mixture is, to be safe, preferably transferred to a third reactor and esterified to a conversion (C3) of generally  greater than 97%.
The reaction mixture obtained is subsequently fractionated continuously into the esterification product and a THF/BDO/water mixture. The THF/BDO/water mixture is fractionated in a column system and recovered 1,4-butanediol is returned to the first esterification reactor. The esterification product is transferred continuously to the precondensation stage.
b) Precondensation Stage
The precondensation stage generally has at least two, preferably at least three, particularly preferably at least four, temperature zones. The temperature of a zone is generally from 1 to 25xc2x0 C., preferably from 1 to 15xc2x0 C., particularly preferably from 1 to 10xc2x0 C., higher than the temperature of the preceding zone. The temperature range for the overall precondensation is generally from 220 to 300xc2x0 C., preferably from 225 to 290xc2x0 C., particularly preferably from 230 to 260xc2x0 C.
In general, the precondensation is carried out in a pressure range from 0.05 bar to the esterification pressure in the last reactor of the reactor cascade of the esterification stage. It is preferably carried out so that the pressure in the first zone corresponds to the reaction pressure in the last esterification reactor, and in the following zones is generally from 20 to 500 mbar, preferably from 25 to 450 mbar, particularly preferably from 30 to 400 mbar, with the pressure preferably dropping from one zone to the following zone.
The precondensation stage is preferably carried out in an ascending tube reactor.
The residence times for the overall stage b) of the process are generally from 10 to 80 minutes, preferably from 15 to 70 minutes, particularly preferably from 30 to 60 minutes. In a particularly preferred embodiment, the precondensation is carried out in four temperature zones, with the temperature rising slightly from zone to zone in the above-described ratios and the pressure being reduced from the first to the fourth zone within the limits described. In this preferred embodiment, the fourth zone comprises an apparatus for separating vapor and liquid phases. In this zone, excess 1,4-butanediol, THF and water are separated from the precondensate.
The catalysts mentioned for the esterification stage of the process of the present invention can likewise be introduced in the amounts specified into the precondensation stage.
After the precondensation b), the precondensate has a viscosity number of generally from 5 to 50 ml/g, preferably from 20 to 50 ml/g, measured as a 0.5% strength by weight solution in phenol/o-dichlorobenzene (1:1) in accordance with DIN 53728, Part 3 (1985), at 25xc2x0 C.
The precondensate is subsequently transferred to the polycondensation reactor (stage c)).
c) Polycondensation Stage
Stage c) is generally carried out in a single zone at temperatures of generally from 240 to 290xc2x0 C., preferably from 240 to 270xc2x0 C., particularly preferably from 240 to 265xc2x0 C. The pressure is generally from 0.2 to 20 mbar, preferably from 0.3 to 10 mbar.
The residence times are usually from 30 to 180 minutes, preferably from 35 to 150 minutes.
During the polycondensation, a renewal of the surface of the product is preferably carried out. Renewal of the surface means that new polymer is continually brought to the surface of the melt so as to aid exit of the diol. This is preferably from 1 to 20 m2/kg of product and minute, particularly preferably from 1.5 to 6 m2/kg of product and minute.
In general, no further catalyst is added in the polycondensation stage, but it is also possible to add a catalyst, for example a catalyst as described above, in this stage of the process, too.
After the continuous polycondensation, the polyester generally has a viscosity number of from 60 to 180 ml/g, preferably from 90 to 160 ml/g, determined in a 0.5% strength by weight solution in a phenol/o-dichlorobenzene mixture (weight ratio=1:1, at 25xc2x0 C.) in accordance with DIN 53728, Part 3 (1985).
In the polycondensation stage of the process of the present invention, lubricants and nucleating agents are preferably added together to the polymer melt when the viscosity number has reached at least 80%, preferably at least 95%, particularly preferably 100%, of the desired final viscosity number of the polyester, and the melt is, if desired, post-condensed and subsequently discharged, cooled and granulated. The lubricant is preferably added in an amount of generally from 0.01 to 3% by weight, preferably from 0.1 to 1% by weight, particularly preferably from 0.2 to 0.8% by weight, and the nucleating agent is added in an amount of generally from 0.001 to 2% by weight, preferably from 0.01 to 1% by weight, particularly preferably from 0.03 to 0.5% by weight, in each case based on 100% by weight of polybutylene terephthalate.
The addition is particularly preferably in the form of a suspension, with the nucleating agent being suspended in the lubricant, if desired at elevated temperature, prior to addition to the melt. Depending on the type of lubricant used, it may be necessary to heat the mixture of lubricant and nucleating agent to generally from 30 to 150xc2x0 C., preferably from 60 to 130xc2x0 C., in order to prepare a suspension and subsequently add it to the polymer melt.
Examples of suitable lubricants are low molecular weight polyethylene waxes which are solid at room temperature and have to be heated to prepare a suspension of the nucleating agent.
Such lubricants are low molecular weight polyethylene waxes which may advantageously contain functional groups such as glycidyl and/or carboxyl groups and have a mean molecular weight Mn (number average) of generally from 500 to 20,000 g/mol, preferably from 1000 to 10,000 g/mol, particularly preferably from 1000 to 5000 g/mol and very particularly preferably from 1000 to 3000 g/mol.
The molecular weight is usually determined by gel permeation chromatography (GPC) using an LDPE standard (low density polyethylene). The melt viscosity is preferably from 100 to 5000 mm2/g, particularly preferably from 100 to 3000 mm2/g and very particularly preferably from 100 to 2000 mm2/g, (in accordance with DIN 51 562) at 120xc2x0 C.
Suitable nucleating agents are, in particular, minerals selected from the group consisting of alkali metal (alumino)silicates and/or alkaline earth metal (alumino)silicates, preferably selected from the group consisting of island silicates or sheet silicates. All possible compounds such as hydroxides, carbonates, hydroxycarbonates, sulfates, silicates and phosphates and phosphonates can be used. Further suitable nucleating agents which may be mentioned are alkali metal or alkaline earth metal salts of organic or inorganic acids, for example sodium antimonate, calcium stearate, sodium terephthalate, calcium citrate and metal acids (basic acids) of titanium or tungsten.
Suitable derivatives of inorganic acids are preferably phosphoric acid derivatives, with particular preference being given to sodium phenylphosphinate, zinc phosphate, calcium bis(3,5-di-tert-bufylethylphosphonate) (Irganox(copyright) 1425 from Ciba Geigy AG) and tetrakis(2,4-di-tert-butylphenyl) 4,4-biphenylenedi-phosphonite.
Suitable polycondensation apparatus are known to those skilled in the art. In a particularly preferred embodiment, it is possible to discharge the melt from the polycondensation reactor, to add the mixture of lubricant and nucleating agent by means of suitable devices, e.g. a metering pump with heating, and subsequently to homogenize the polymer melt in a static mixer and then discharge, cool and granulate it.
The polybutylene terephthalate obtained generally has an acid number of  less than 50 meq/kg, preferably  less than 35 meq/kg, particularly preferably  less than 30 meq/kg. The acid number was determined by titration with aqueous sodium hydroxide.
The process of the present invention has the advantage that only small amounts of THF are formed from 1,4-butanediol, and thus only little 1,4-butanediol is lost. In general, the amount of THF formed, based on the amount of polybutylene terephthalate obtained, is  less than 5% by weight, preferably  less than 4% by weight, particularly preferably  less than 3.5% by weight. Likewise, only little 2,5-dihydrofuran is formed from 1,4-butanediol. The amount of 2,5-dihydrofuran formed is preferably  less than 150 ppm, particularly preferably  less than 100 ppm, based on the amount of polybutylene terephthalate obtained.